Media such as compact disc read-only memory (CD-ROM) or digital video disc (DVD) is read by a device which has a laser in a drive head. The laser scans or reads the disc as the device's drive unit spins the disc at a high rate of speed. The drive unit typically comprises a spindle motor with a turntable that supports the disc. Modern CD drive units operate at a constant angular velocity (CAV). In CAV mode, the speed of the spinning disc remains constant regardless of where the laser is reading data on the disc. Thus, the inner tracks on the disc read at a lower transfer rate than the outer tracks on the disc.
In contrast, DVD drive units may need to operate at a constant linear velocity (CLV) as well as CAV. The rate of rotation for CLV mode is designed to be less than that for CAV mode. In CLV mode, the spindle motor decreases the rate of rotation of the spinning disc as the drive head moves to read data at the outer tracks of the disc. Thus, when a drive unit is operating in CLV mode, the speed of the spindle motor is constantly changing since data from every track will be transferred at the same speed when the laser reads it.
Drive units which are capable of reading both CD and DVD media should be able to operate in both CAV and CLV modes. As CD drive speeds have increased (24.times., 32.times. and beyond), the rate of rotation has also increased. Currently, the rate of rotation for CAV mode CD drives is approximately 6800 rpm. This high rate of speed has caused a significant problem in the industry because typical CD media is not perfectly balanced. When drive units spin even slightly unbalanced media at high rates of speed, vibration and noise are created. Such undesirable characteristics are unacceptable to consumers and may affect the long term reliability of the drive unit. As a result, high speed CD drive units typically utilize an auto-balancer spindle motor. This improved motor has several ball bearings which are free to move along a circular rail around the spindle motor. As the drive unit spins with unbalanced media, the ball bearings automatically move to counter-balance the imbalance.
Although auto-balancer spindle motors eliminate media imbalances, it has only been successful for CAV mode CD drives which operate in excess of 20.times. (approximately 4600 rpm). When the auto-balancer feature is used with CLV drives, the constantly changing speed of the motor never allows the ball bearings to find the proper position to perfectly counter-balance the unbalanced media, thereby exacerbating the vibration problem. Thus, there is a need for a drive unit which is capable of operating in both CAV and CLV modes without excessive vibration.